The invention relates to a backlight module, and in particular to a backlight module comprising a positioning structure for a light guide plate therein.
Currently, TFT-LCDs (thin film transistor-liquid crystal display) are increasingly applied in various electronic devices, such as personal digital assistants (PDAs), notebooks, computers, and TVs. The liquid crystal panel modules require a backlight module as a main light source. The backlight module is disposed behind the liquid crystal panel. Larger screens require light guide plates of the backlight module to be larger. The engagement method of the light guide plate thus has become an important factor in assembly of the backlight module.
FIG. 1A is a partial side view of a conventional backlight module 10, comprising a frame 11, a light guide plate 12, at least one light source 13, a plurality of optical films 14, and a reflecting sheet 15. The light guide plate 12, the light source 13, and the optical films 14, such as light diffusion sheet 141, and prism sheets 142, are supported and covered by the frame 11. The light source 13 is surrounded by the reflector 131 such that light emitted from the light source 13 is concentrated toward the light guide plate 12. The light guide plate 12 is disposed between the optical films 14 and the reflecting sheet 15. The light guide plate 12 directs the light emitted from the light source 13, located at a side of the light guide plate 12 forward, as indicated by the arrow, such that the light is uniformly transmitted, and subsequently distributed by the optical films 14 to produce a required color. FIG. 1A eliminates the panel, upper, and lower covers to clearly show the internal structure of the backlight module 10.
FIG. 1B is a top view of a conventional backlight module 10A with a positioning structure for light guide plate 12a, with other elements eliminated. 14a is the optical film located above the light guide plate 12a. 
The light guide plate 12a of the conventional backlight module 10A is positioned by glue 16, fixing an edge 120 thereof on a side of the frame 11a by glue. The positioning method, however, has the following disadvantages. Since the light guide plate 12a is glued to the frame 11a, it is difficult to reassemble. If the glued portion is uneven, the light guide plate 12a can easily dislodge from the frame 11a. Additionally, when the backlight module 10A is inspected by reliability assurance (RA) or thermal shock test (TST), temperature changes can generate thermal expansion and contraction. Since glued edge 120 of the light guide plate 12a is fixed and unable to expand or contract accordingly, the light guide plate 12a may be displaced, causing unevenness. Thus, the backlight module 10A may have difficulty passing reliability assurance (RA) or thermal shock testing (TST).
FIG. 1C is a schematic view of another conventional backlight module 10B with a positioning structure for a light guide plate 12b. 14b is optical film disposed above the light guide plate 12b. 
As shown in FIG. 1C, the light guide plate 12b of the conventional backlight module 10B comprises two symmetrical protruding portions 121, 122. The light guide plate 12b is positioned by engaging the protruding portions 121, 122 with the frame 11b. A disadvantage is that the frame 11b must be shaped corresponding to the protruding portions 121, 122, and thickness W1 and W2 of two sides of the frame 11b reduced. Structural strength is significantly affected. Additionally, the protruding portions 121, 122 fix the light guide plate 12b only along X and Y axes, but not along Z axis. Thus, if the liquid crystal display is moved in the direction Z, the light guide plate 12b can easily be displaced. Moreover, the additional shape of the protruding portions 121, 122 enlarges the size of the light guide plate 12b, thereby increasing total weight.
FIG. 1D is a schematic view of a conventional backlight module 10C with a positioning structure for a light guide plate 12c. 14c is disposed above the light guide plate 12c. 
As shown in FIG. 1D, the light guide plate 12c of the conventional backlight module 10C comprises protrusions 123 and 124, inserted in a groove (not shown) of the frame 11c. Thus, the light guide plate 12c is securely positioned and inserted in the frame 11c. Manufacture of the groove and protrusions is, however, costly and complicated, and consequently, the detachment of the light guide plate is time-consuming. Moreover, the protrusions 123 and 124 extend along the Y axis. Thus, the light guide plate 12c cannot be fixed in direction Y. When rotating the liquid crystal display, insufficient structural support often produces unpleasant noises.